Workpieces, including food products, are cut or otherwise portioned into smaller portions by processors in accordance with customer needs. Also, excess fat, bone, and other foreign or undesired materials are routinely trimmed from food products. It is often desirable to portion and/or trim the workpieces into uniform sizes, for example, for steaks to be served at restaurants or chicken fillets used in frozen dinners or in chicken burgers. Much of the portioning/trimming of workpieces, in particular food products, is now carried out with the use of high-speed portioning machines. These machines use various scanning techniques to ascertain the size, weight and shape of the food product as it is being advanced on a moving conveyor. This information is analyzed with the aid of a computer to determine how to most efficiently portion the food product into optimum sizes. For example, a customer may desire chicken breast portions in two different weight sizes, but with no fat or with a limited amount of acceptable fat. The chicken breast is scanned as it moves on an infeed conveyor belt and a determination is made through the use of a computer as to how best to portion and/or trim the chicken breast to the weights desired by, and fat content level acceptable to, the customer, so as to use the chicken breast most effectively.
Portioning machines of the foregoing type are known in the art. Such portioning machines, or components thereof, are disclosed in prior patents, for example, U.S. Pat. Nos. 4,962,568 and 5,868,056, which are incorporated by reference herein. As typical, the portioning machine includes an infeed conveyor having a moving, solid belt that slides over a support structure. The infeed conveyor belt is driven at a selected speed by a drive motor. The drive motor can be composed of a variable speed motor to thus adjust the speed of the infeed belt. The workpieces are first carried by the infeed conveyor past a scanning station, where the workpieces are scanned, for example by an optical scanner, to ascertain selected physical parameters, for example, their size and shape, and then determine their weight, typically by utilizing an assumed density for the workpieces. In addition, it is possible to locate discontinuities (including voids), foreign material, and undesirable material in or on the workpiece, for example, bones or fat in a meat portion.
The data and information measured/gathered at the scanning station are transmitted to a computer, preferably on board the portioning apparatus, which records the location of the workpiece on the infeed conveyor as well as the shape and other parameters of the workpiece. With this information, the computer determines how to optimally cut, portion and/or trim the workpiece at the portioning station. Once the workpieces are scanned, they may be transferred to a cutting conveyor, typically composed of a metal mesh material. Portioning may be carried out by various types of cutting/portioning devices.
The cutting devices rely on the determined location of the workpiece on the belt at the scanning station to know where to make the cuts determined by the computer. Thus, it is important that the speed of the infeed conveyor and the cutting conveyor 8 be accurately controlled, and ideally both conveyors will operate at the same uniform speed. If the speeds of the conveyor(s) vary, then the actual position of the workpiece relative to the cutting/portioning devices will not correspond to that calculated by the computer.
As noted above, the cut paths are computed from the scan data and product specifications to create the desired portions. The scanning and cutting/portioning stations are typically separated by a distance on the order of 3 meters. In order to achieve the desired cutting result, the transporting of the work product on the cutting conveyor needs to be extremely accurate relative to its predicted position. A position error of even 1 mm of the work product on the conveyor creates an unacceptably inaccurate cut of the work product.
Cutting belts are often driven by sprockets mounted on a drive shaft. The sprocket teeth engage the links of the belt or the links of a chain connected to the belt. The angular position of the belt drive shaft is known from a digital signal provided by an encoder that monitors the angular position of the drive shaft. If the link pitch of the belt was always constant throughout the belt, it would be easy to convert the drive shaft position to belt position. However, the belt pitch is often not very consistent due to manufacturing inaccuracies or wear and damage that occur over time. The metal belt links typically rotate on metal cross rods. See for example links 30 and cross rods 32 in FIG. 3 herein. The fit between the rods and holes in the links is necessarily loose, and as wear occurs, the holes become elongated to form slots, and the pitch of the belt links changes. If some part of the belt gets damaged and is replaced with a new belt, there will be two different pitches of belt within the system. A position error of 1 mm in 3 meters on a belt with a link pitch of 14 mm can be created by wear of only 4.7 microns (1.8 ten thousandths of an inch) per link. As a reference, a roller chain (such as on a bicycle) is generally allowed to wear up to 125 microns per link before replacement; so the assumption of uniform belt pitch on a conveyor belt could cause very large errors in the assumed position of a work piece on the conveyor belt.
One means of overcoming this issue is to measure belt position using an encoded friction roller that rolls against the belt surface. In this regard, see U.S. Pat. No. 7,025,196, which is incorporated by reference herein. The encoder signal can either be used directly to time the cutters or it can be used as feedback to the drive shaft to keep it at a constant speed. There are significant disadvantages to this approach, including difficulties maintaining friction between the friction roller and belt without excessive belt tension and in the presence of contaminants such as chicken, chicken fat, and water on the belt. Wrapping the belt path partially around the encoded friction roll could reduce slippage issues, but does not necessarily eliminate them.
The present disclosure seeks to address the need to accurately and reliably know the location of a workpiece on a conveyor.